Using a highly sensitive and specific radioimmunoassay of secretin, roles of secretin in physiological and pathophysiological states in man and dogs with pancreatic fistula will be studied. The relationship between the release of secretin and the amount of acid delivered during normal alimentation, the role of secretin in physiological dose range on exocrine pancreatic secretion and possible roles of agents other than acid on the release of secretin will be studied. The agents to be studied include bile, sodium oleate, somatostatin, endephalin and endorphin. The role of the vagus nerve on release of secretin and the action of secretin in the exocrine pancreas will be studied. The release of secretin and action of secretin in various gastrointestinal disorders including peptic ulcer disease, Zollinger-Ellison Syndrome, achlorhydria, adult celiac sprue, exocrine pancreatic insufficiency and patients with truncal vagotomy or parietal cell vagotomy will be studied. Lastly, a possible physiological role of secretin on inhibition of gastrin release will be investigated. Radioimmunoassay of secretin and gastrin, exocrine pancreatic secretion, gastric secretion, peptic ulcer, Zollinger-Ellison Syndrome, achlorhydria, adult celiac sprue, exocrine pancreatic insufficiency, truncal vagotomy, parietal cell vagotomy, somatostatin, enkaphalin, endorphin, bile, sodium oleate.